How to use the MVVM RelayCommand Attribute?

Question

I have been trying out attributes and ran in an issue trying to use the RelayCommand attribute from the Community Toolkit.

I looked over the .NET Documentation on RelayCommand Attribute and tried to implement a simple example as follows:

using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;

namespace HelloWorld_WinUI3.ViewModels;

public partial class MainViewModel : ObservableObject
{
    public MainViewModel()
    {
    }

    private int _counter = 0;

    [RelayCommand]
    private async Task DoSomethingAsync()
    {
        await Task.Delay(1000);
        MyString = $"Hello {_counter++}";
    }

    [ObservableProperty]
    private string _myString = "Hello"; 
}

This produces the error: CS0616 'RelayCommand' is not an attribute class

Can someone explain what is wrong with this code and how to use the [RelayCommand] attribute properly?

score 2 · Accepted Answer · answered Apr 07 '23 at 01:56

2

The CommunityToolkit.Mvvm works with WinUI 3 and.NET7. Actually, I use it a lot, including every sample I create for StackOverflow answers.

I guess, you might be facing this known issue.

So, at this moment, try the v8.0 instead of v8.1.

answered Apr 07 '23 at 01:56

Andrew KeepCoding

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Turns out I was using 7.1.2. Updating it to 8.0.0 or 8.1.0 fixed the issue for me. Thanks. – JasonC Apr 12 '23 at 18:13

score 1 · Answer 2 · edited Apr 17 '23 at 11:50

Using MVVM Source generators is still problematic for WinUI 3; I suggest using raw Relay command declaration:

public class MyViewModel : ObservableObject
{
    public MyViewModel()
    {
        IncrementCounterCommand = new RelayCommand(IncrementCounter);
    }

    private int counter;

    public int Counter
    {
        get => counter;
        private set => SetProperty(ref counter, value);
    }

    public ICommand IncrementCounterCommand { get; }

    private void IncrementCounter() => Counter++;
}

score 0 · Answer 3 · answered Apr 06 '23 at 21:47

It appears the CommunityToolkit does not fully support WinUI3 or .NET 7 at this time Docs.

Running the same example on a WPF project works fine.

[ObservableProperty] seems to work in a WinUI3 project
[RelayCommand] does not work with WinUI3 project.

How to use the MVVM RelayCommand Attribute?

3 Answers3